1. Field of the Invention
The present invention relates to ceramic substrates having a polished surface zenith high surface smoothness and methods of polishing such ceramic substrates, and more particularly, to a method of polishing a ceramic substrate for use in a thermal fixation device for a toner image such as a copying machine and a printer.
2. Description of the Background Art
Conventionally, when a ceramic material is used for various purposes, in general. the surface of the ceramic material should be polished or ground into smoothness. There have been various proposed methods of smoothing a ceramic surface depending upon the shape, use, required smoothness and the like of the ceramic material.
One typical method of polishing a ceramic material is barrel-polishing, according to which a ceramic material and an abrasive are put together into a container to polish the ceramic material by rotation or vibration, and Japanese Patent Laying-Open No. 58-192745 for example discloses a method of polishing a ceramic element by a vibrating barrel using a pier-shaped abrasive.
Other methods of treating a ceramic surface include lapping, honing, and grinding. These methods employ hone or abrasive grains to pressurize the ceramic material to be treated and the surface is ground.
The conventional, typical methods of polishing or grinding described above are suitable for treating relatively small and thick ceramic elements, but are not appropriate for smoothing the surface of a ceramic substrate having a large area and a relatively small thickness such as a substrate for a ceramic heater in a thermal fixation device for toner image.
In barrel-polishing, for example, a thin ceramic substrate is sometimes destroyed by a grinder during rotation or vibration. In lapping, honing, and grinding, a ceramic substrate is prone to cracking, because a prescribed pressure is applied between abrasive grains or a grinder used and the ceramic material.
The lapping, honing, grinding or the like requests that the untreated surface must be ground as much as 0.1 to 0.2 mm in order to eliminate variations in the surface smoothness by the working. Therefore, a ceramic substrate having a thickness larger than a finished product by the margin for working should be prepared, which increases the material cost.
The present invention is directed to a solution to the problem, and it is an object of the present invention to provide a ceramic substrate having a large area, a relatively small thickness and a smooth surface such as a ceramic substrate for use in a ceramic heater in a thermal fixation device for toner image and to provide a method of polishing the surface of such a ceramic substrate having a large area and a relatively small thickness into smoothness without damaging the surface.
In order to achieve the above-described object, a method of polishing a ceramic substrate according to the present invention uses a ductile rotating body containing abrasive grains, and the surface of the ceramic substrate is polished by the circumferential portion of the rotating body. This polishing method is preferable for polishing a thin ceramic substrate, particularly, a ceramic substrate as thin as 20 mm or less.
In the method of polishing a ceramic substrate, the direction orthogonal to the rotating axis of the rotating body is preferably inclined by an angle in the range from 10xc2x0 to 80xc2x0 relative to the direction of polishing the ceramic substrate. If this angle is smaller than 10xc2x0 or larger than 80xc2x0, a line is impressed on the ceramic substrate by the abrasive grains, and the surface roughness is relatively increased.
The polishing process may be divided into two or more steps and the average grain size of abrasive grains contained in the rotating body may be reduced stepwise.
A ceramic substrate resulting from the polishing as described above has at least one surface polished, which surface is formed by a substantially flat portion and a recessed portion remaining in the flat portion.
Such a ceramic substrate is suitable for a ceramic substrate having a large area and a relatively small thickness such as a substrate for a ceramic heater used in a thermal fixation device for toner image. The flat portion herein includes microscopically small irregularities.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.